JP-2011-32929A describes a link apparatus in which a roller is engaged with a cam groove. The link apparatus will be described with reference to FIG. 7. In the link apparatus shown in FIG. 7, the same parts and components as those in embodiments of the present disclosure, which will be described later, are indicated with the same reference numerals.
According to JP-2011-32929A, a low-pressure-loop exhaust gas recirculation (LPL-EGR) adjusting valve 5 is driven by an electric actuator 6 configured with an electric motor 43 and a gear reduction mechanism 44. A link mechanism 8 varies an output characteristic of the electric actuator 6 to drive an intake throttle valve 7.
The link mechanism 8 is comprised of: a cam plate 14 provided in a cam groove 13 in such a manner as to rotate along with the LPL-EGR adjusting valve 5; and a lever 16 which rotates along with the intake throttle valve 7 and has a groove-engaging body 15 engaged with the cam groove 13. The groove-engaging body 15 has a roller 15a and a pin 15b. 
The pin 15b is a cylinder-shape shaft body provided in the lever 16, and is inserted through a penetration hole formed in a center of the roller 15a. 
The roller 15a is rotatably supported on the pin 15b and rotates in the cam groove 13.
The roller 15a is prevented from being dropped from the pin 15b by a roller holder. The roller holder is provided in a distal end portion of the pin 15b, such as a diameter expansion portion, a circlip, or an E-shape ring.
When the roller holder is disengaged from the pin 15b, it is likely that the roller 15a may be dropped from the pin 15b. 
When the roller 15a is dropped from the pin 15b, it is likely that the dropped roller 15a is involved into a moveable member such as gear. In this case, the moveable member may be locked (fixed) by the roller 15a. 
Thus, when the roller holder is disengaged from the pin 15b, a malfunction may occur due to the dropped roller 15a. 
In addition, other link mechanism in which the roller 15a is supported by the pin 15b may have the same issue.